There are a number of devices that are moisture sensitive. A particular class of devices is an organic light-emitting diode device (OLED). Such devices commonly include a substrate, an anode, a hole-transporting layer made of an organic compound, an organic luminescent layer with suitable dopants, an organic electron-transporting layer, and a cathode. OLED devices are attractive because of their low driving voltage, high luminance, wide-angle viewing, and capability for full-color flat emission displays. Tang et al. described this multilayer OLED device in U.S. Pat. Nos. 4,769,292 and 4,885,211.
A common problem with OLED displays is sensitivity to moisture. Typical electronic devices require humidity levels in a range of about 2500 to below 5000 parts per million (ppm) to prevent premature degradation of device performance within a specified operating or storage life of the device. Control of the environment to this range of humidity levels within a packaged device is typically achieved by encapsulating the device or by sealing the device and a desiccant within a cover. Desiccants such as, for example, molecular sieve materials, silica gel materials, and materials commonly referred to as Drierite materials, are used to maintain the humidity level within the above range. Particular highly moisture-sensitive electronic devices, for example, organic light-emitting devices (OLED) or panels, require humidity control to levels below about 1000 ppm and some require humidity control below even 100 ppm.
Numerous publications describe methods or materials for controlling humidity levels within enclosed or encapsulated electronic devices. Kawami et al., in U.S. Pat. No. 5,882,761, has taught the use of a desiccant layer over the organic layers of an OLED display between the substrate and the top seal. Boroson et al., in U.S. Pat. No. 6,226,890, disclose the use of a castable blend of desiccants with a suitable binder. Kim et al. in US 2003/0127976 A1 teach the use of two sealants surrounding an OLED device. While this can be a way to reduce the likelihood of sealant failure, it is no more effective at reducing moisture transmission rate into the device than would be a single, wider sealant. Wang et al. in US 2003/0122476 A1 show the use of two seals surrounding an OLED device with a desiccant between the two seals, but no desiccant inside the enclosure that contains the OLED device. Peng in U.S. Pat. No. 6,589,675 B2 also teaches the use of two seals with a desiccant between them. It is also known to seal an OLED device with a metal seal, for example as described in U.S. Pat. No. 7,202,602.
The methods described in the above references rely on the seal between the substrate and the enclosure to limit the rate of water permeation and require sufficient volume within the enclosure to hold enough desiccating agent to absorb moisture for the entire lifetime of the device. These limitations can require very wide seals, and thus wide borders beyond the display area, to prevent the rate of moisture permeation from exceeding the rate of moisture absorption by the desiccating agents. Such an arrangement can require large or deep enclosures to hold sufficient quantities of desiccating agent. For top emitting OLED devices in particular, this enclosure size requirement can be a significant issue since most desiccating agents are not transparent, and therefore cannot be located over the emitting areas of the OLED. For top emitting OLED devices, desiccating agents must typically be placed outside the display area but still inside the enclosure, resulting in large borders beyond the display area. It is desirable to maintain small borders beyond the display area of OLED devices to minimize the size of a given device and to maximize the number of devices produced on a given mother glass substrate during manufacturing.
Therefore, there still remains the need to reduce moisture transmission rate into highly moisture-sensitive devices, such as OLED devices, in a way that does not add to the size of the border required beyond the display area of an OLED device.